It does happen from time to time. The buyer really has a responsibility to secure the animal in a safe manner for the trip home, both to protect the animal and to protect other motorists.
If a cow can get its head over a fence or other barrier, it will think it can jump over it.
And, there are just some animals and some breeds that are more hyper than others. Who knows how the animal was treated both before and during the sale, too? It could just be an excitable animal or it could have been mishandled.
Rafter L Murray Greys